Method for transmitting signals in a radio communication system

ABSTRACT

The invention relates to a method for transmitting signals in a radio communication system consisting of at least two frequency signal-transmitting channels. At least the first (fcoord) of said frequency channels is used for organising radio resources, and at least the second (fi) of the frequency channels is used for transmitting signals between the stations of the system where the signal transmission is carried out by the frequency channels from/in a direction of said stations in conformity with a predefined temporal pattern (P 1 , P 2 , P 3 ).

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based on and hereby claims priority to InternationalApplication No. PCT/EP2004/001707 filed on Feb. 20, 2004 and GermanPatent Application No. 10307809.6 filed on Feb. 24, 2004, the contentsof which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

The invention relates to a method for transmitting signals in a radiocommunication system, in particular in a mobile telephone system.

Future radio communication systems will support very high data rates inorder to be able to operate multimedia applications with the necessaryquality of service. A continued increase in subscriber numbers is alsoto be anticipated such that further frequency bandwidths must bedeveloped for use by radio communication systems. In order to makeefficient use of these frequency bandwidths however radio communicationsystems need to operate over a large frequency range.

Various methods are used in radio communication systems for segmentingresources and multiplexing. In addition to Time Division Multiplexing(TDM) and Code Division Multiplexing (CDM), various frequency channelsare created through Frequency Division Multiplexing (FDM). The FDMmethod involves segmenting a wide frequency spectrum into many separatefrequency channels, each with a narrow bandwidth, within the frequencyrange, thus creating a frequency channel grid defined by the gapsbetween the carrier frequencies. Advantageously this allows a pluralityof subscribers on different frequency channels to be servedsimultaneously and resources to be better tailored to individualsubscriber requirements. A sufficient distance between frequencychannels ensures that interference between the channels can be reducedand controlled.

In order for the narrow band frequency channels to be used by acorresponding access method known as FDMA (Frequency Division MultipleAccess), the transmitter and receiver must each select a correspondingcarrier frequency in a coordinated manner. Before using thecorresponding resource, i.e. the radio channel, continued checks need tobe made to establish whether the selected resource is not already beingused by other stations. The resource is then reserved, if necessary thereservation being communicated to other potential stations so that thesestations do not subsequently access the resource at the same time andcause collisions. The challenge here is to optimize the efficient use ofthese frequencies, using few resources and as far as possible only onetransmitter and receiver (transceiver) for each terminal. A furtherconstraint is that all stations have equal rights i.e. no stationassumes a control function for assigning the frequency channels via aplurality of stations. In particular in self organizing networks andnetworks without an infrastructure, so-called ad-hoc networks, stationswith equal priority frequently perform the same algorithms andprotocols. A known example for such networks is the local wirelessnetwork (Wireless LAN) according to the IEEE 802.11.standard.

If stations in networks such as these are not informed about the use offrequency channels because the appropriate information is not collatedand distributed via a central station, a station will be unable todecide which frequency channel it should propose or select forcommunicating with another station. Also, a station will not know whenanother station is ready to receive and on which frequency.Consequently, a station would generally be unable to establish aconnection with another station if both were free to select thefrequency channels. Also, a distribution service (broadcast) on onefrequency would only reach one part of the station which happened to beready to receive on this frequency.

The requirement is therefore to coordinate the use of these orthogonalresources. If a particular frequency is to be used for this purpose, atfixed, predetermined times i.e. all stations are ready to receive onthis frequency, then the frequencies which exist alongside them cannotbe used. These resources would then be unused and unavailable for thesestations.

In existing cellular mobile telephone systems, the frequencies areallocated to the mobile stations (MS) located within one radio cell ofthe base station via a central station, the base station (BS). With GSM(Global System for Mobile Communication) for example, one centralfrequency channel per radio cell is used for transmitting generalinformation used by the mobile stations for determining the frequencychannel for logon and request of resources. If a mobile station wishesto transmit data, it requests the data from the base station on thefrequency channel it knows. The base station then notifies the mobilestation of the appropriate carrier frequency on which it can communicatewith the base station. The allocation and management of availableresources is controlled centrally in a Base Station Controller (BSC)subordinating the base station and signaled from the base station. Thesame applies to third generation UMTS (Universal MobileTelecommunication System) systems which also use a base station tosignal the frequency allocation according to this central principle.

Because of the central control, this approach cannot be used in adecentrally organized system without a central entity. Other systems,such as for example WLAN systems (Wireless Local Area Networks)according to the HIPERLAN Type 2 Standard known for example from theETSI/BRAN document “Broadband Radio Access Networks (BRAN); HIPERLANType 2 Functional Specifications Data Link Control (DLC) Layer; Part4—Extension for Home Environments., draft DTR/BRAN-0020004-4, ETSI,Sophia Antipolis, France, April 2000 or according to the IEEE 802.11standard, use only one carrier frequency for communicating. Switching toa different frequency serves to avoid interference. If a new, usablecarrier frequency is found, all stations transmit and receive on thisfrequency. In HIPERLAN/2 this method is known as Dynamic FrequencySelection (DFS). A simultaneous needs-based use of several frequenciesby any stations to increase the total possible capacity of the system orof an individual connection is however not provided. Consequently, themaximum date rate is limited to one frequency channel.

Similar to the DFS method according to the HIPERLAN/2 Standard, a systemis proposed in the document by R. Sakata, K. Naha, H. Murata, S. YoshidaPerformance Evaluation of Autonomous Decentralized Vehicle-groupingProtocol for Vehicle-to vehicle Communications, in Prov. IEEE VTC,Boston, Mass., Sep. 24-28, 2000 pp 153-157, in which vehicles formgroups using different frequencies. In this method, neighboring vehiclesshare the same carrier frequency. By measuring the active frequencychannels, it is possible to participate simultaneously in a plurality ofgroups and switch groups to take account of the changing networktopologies. This assumes that, generally speaking, only one frequency isused for exchanging data whereas the other frequencies only receive soas to prepare for occupancy and a possible change of frequency. In orderto be able to receive simultaneously on other frequencies, it isproposed that two transceivers be used, one for data exchange and thesecond for measuring other potential frequencies. The use of a pluralityof available frequencies for communicating with neighboring stations forthe exchange of data is however not described, even here.

The DECT cordless telephone standard (Digital Enhanced CordlessTelephone) specifies a flexible use of resources including differentfrequency channels. The method by which the resources are used is calledDynamic Channel Selection (DCS). Although in this system, when inso-called “Basic Mode”, one base station has exclusive control overresource allocation, the relaying of data packets is supported. Toachieve this, as in a distributed, decentralized system, resourceavailability is checked and occupied.

For each carrier frequency, DECT specifies a frame with 24 time slots,12 time slots being used for the downlink and 12 for the uplink with afixed allocation in the Time Division Duplex method (TDD), see FIG. 1.

DECT is thus an FDMA/TDMA/TDD system in which 10 carrier frequencies cangenerate up to 120 communication (bearer) channels. Before setting up aconnection, a station measures different communication channels. Theresulting Received Signal Strength Indicator (RSSI) is recorded in atable. All channels with a signal level lower than the lowest level (−93dBm) are classified as “quiet” and may be used to set up a communicationchannel. The upper threshold, which is classified as “busy” defines therange of busy channels and is variable. As a rule the upper threshold isset to −33 dBm. Channels with signal levels in excess of this thresholdmay not be used for setting up a communication channel.

The maximum number of channels in a cell depends on the number of basestation transceivers. If only one transceiver is available per basestation, only one mobile station per time unit (time slot) can beoperated. A free allocation of time slots for direct communicationbetween mobile stations in parallel with the communication between amobile station and base station is not defined in greater detail. DECT,however, supports this “walkie-talkie” mode. As soon as a mobile stationwith only one transceiver occupies a channel, the remaining frequencychannels parallel to this time slot are marked as so-called blind slots,see FIG. 1. Stations may neither transmit nor receive on these blindslots and conduct measurements of the received signal level. Dependingon how quickly the stations are able to switch from transmitting toreceiving and vice versa (Transceiver Turn-Around), time slots beforeand after are also to be marked as blind slots. The base stationperiodically transmits control information to all mobile stations viathe currently busy time slots. This information allows the mobilestations to measure the remaining unoccupied time slots which can beused as potential candidates for a future communication between the basestation and mobile station. Owing to the base station's special role,the methods described and implemented in the DECT system are notapplicable to a target system in which no central entity exists but inwhich simultaneous use of time slots on different frequencies is to befacilitated.

SUMMARY OF THE INVENTION

One possible object of the invention is thus to enable a plurality ofavailable frequency channels to be used in a decentrally organizedsystem as efficiently as possible. The inventors propose a radiocommunication system having at least two frequency signal-transmittingchannels, which uses at least the first of said frequency channels fororganising radio resources. The system uses at least the second of thefrequency channels for transmitting signals between the stations of thesystem. Signal transmission is carried out by the frequency channelsfrom/in a direction of said stations in conformity with a predefinedtemporal pattern.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and advantages of the present invention willbecome more apparent and more readily appreciated from the followingdescription of the preferred embodiments, taken in conjunction with theaccompanying drawing of which:

FIG. 1 shows a temporal structure of a known DECT frame and

FIG. 2 shows an exemplary frequency selection based on frequencypatterns.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings, wherein like reference numerals refer to like elementsthroughout.

The inventors propose that the allocation of resources, in particularthe use of different frequencies, be achieved using the FDMA method withperiodically recurring control channels. In this way, a station can useall frequency channels during a first time interval, and must switch toa particular frequency during a second time interval. A control channelallocated to this specific frequency serves to ensure a coordinatedexchange of the relevant control messages and protocol operations aswell as administering the available radio resources, for example inaccordance with the so-called Radio Resource Management (RRM).

The proposed method defines so-called Frequency Patterns as specifiedfor example in FIG. 2. These Frequency Patterns employ a so-calledExchange Phase on the coordination frequency fcoord and a co-calledTransmission Phase on one or a number of random carrier frequencies fi,which follow on from each other.

During the exchange phase, a station receives and/or transmits thecoordinating frequency fcoord on a fixed frequency. This frequency isknown to the stations of a network. On this frequency, for example, astation announces reservation requests, sends out so-called beacons forthe network organization and/or transmits relevant or time criticalinformation to neighboring stations.

In the time remaining, the transmission phase, a station can make freeuse of available frequencies and only needs to take account of orcoordinate its own frequency channels selected on a temporal basis whentransmitting with other stations. This phase can continue to be used formeasuring frequency channels with a view to possible later use.

According to an embodiment of the invention, the phases of the frequencypattern are equidistant in time. This gives rise to at least threefrequency patterns with alternating phases and different sequences ofthe exchange phase and transmission phase as represented in FIG. 2.

If for example a station selects frequency pattern P1, it will startoperating on coordination frequency fcoord in frame 1 and then switchesfor two successive transmission phases, respectively frame 2 and 3, toone or more arbitrary frequencies fi, and afterwards switches back inframe 4 and 1 to the coordination frequency fcoord. In order tointegrate the method according to the invention into existing systems,the phases correspond to a respective frame duration which for examplein the UTRA TDD standard is 10 ms. The pattern repeats itself after fourframes 1 to 4 and the frequency selection procedure starts from thebeginning. Stations belonging to other patterns P2 and P3 represented byway of an example, switch to the coordination frequency fcoord atdifferent times or frames, for example for P2 in the first 1 and thirdframe 3 and for P3 in the third 3 and fourth frame 4. In such aconfiguration there are common times or frames where stations belongingto different frequency patterns are jointly using the coordinationfrequency fcoord e.g. frame 1 for the patterns P1 and P2 (coordinationon common frequency). It is therefore guaranteed that two frequencypatterns always have a common phase within four sequential phases.

According to one embodiment of the invention, it is further proposedthat exactly three frequency patterns be used. This guarantees a maximumdelay of four frames for the exchange of any data between stationsalthough the stations are free to select and use all frequencies. Thismeans that any station can be reached after no more than four frames,irrespective of which frequency pattern it uses. This is of particularinterest if quality of service is to be guaranteed in a radio network.In particular, this is the smallest number of frequency patterns whichenables all frequencies to be used at any time. A station can select oneof these patterns. The corresponding pattern is selected depending onhow many stations have already selected a pattern or how much capacityis still available in the coordination phase and with which stationscommunication is being attempted. If all transmission resources in agroup are occupied by corresponding frequency patterns, anotherfrequency pattern will be selected which may provide additional freeresources that exist during the coordination phase of the previouslyselected pattern. Naturally the invention also supports more than threefrequency patterns with the same advantages already described.

Advantages of the proposed method are on the one hand the simplicity ofthe complexity requirements on the stations' transmitters and receivers(transceivers). As a plurality of frequencies need to be operated, theswitching times between frequencies as well as the switching timesbetween transmission and reception and vice versa should be as short aspossible. Moreover, only one transceiver per station is required for thestations to be able to use all the frequency channels within radio rangesimultaneously.

Furthermore advantages also follow from the economy in decoding andresource management. A station only needs to decode the time slot duringthe coordination phase as it is only at this time that it can assumethat other stations are ready to receive and that broadcast messages arebeing recorded. This means that the occupation of resources andreservation requests can be announced during this time therebyadvantageously facilitating the administration of resources.

A further advantage follows from an energy saving implicitly associatedwith the method. As a station only needs to be ready to receive duringthe coordination phase, it can switch to a sleep mode for the rest ofthe time. A comparable method for saving power for only one carrierfrequency is described in Y.-C. Tseng, C.-S. Hsu, T.-Y. Hsieh.“Power-saving protocols for IEEE 802.11-based multi-hop ad hoc networks”in Proc. IEEE INFOCOM'02 New York, 23-27 Jun. 2002. This defines a timepattern comprising two phases. During the first phase the station is onstandby to receive whereas in the other so-called sleep phase it turnsthe receiver off so as to save power.

The method is accompanied by an efficient use of available frequencychannels. The method permits all frequencies to be used at all times.From a station's perspective, this freedom of choice is only restrictedduring the coordination phases. As however various different patternsare available, stations can transmit data at any time on any frequencyusing a corresponding pattern.

The organization of the frames brings about a virtual fragmentation ofuser groups using different frequencies. If for example different groupsof stations are using different frequencies, although all the stationsare physically located within the decoding range, the stations areseparated as if they could not be reached directly. Although thestations are subdivided into smaller groups and thus have greatertransmission capacity available, their availability in the networkfalls. The broadcast is also virtually fragmented i.e. although allstations could be reached, only some of the stations are receiving.Multicast/broadcast messages therefore need to be transmitted in severalparts (fragments) until all stations have received the correspondinginformation.

As a broadcast is intended to reach as many subscribers as possible, afragmentation into different groups constantly using differentfrequencies would run counter to this objective. Communication withsubscribers belonging to another group constantly using a particularfrequency channel can thus only be achieved by switching frequency andpossibly also by expensive routing methods. Complexity would thus becomparable in that some stations were outside the radio range of otherstations i.e. it would be comparable with a partially meshed network. Atthe same time however, the challenges of such a network also need to bemanaged, such as for example multi-hop transmission. At least onestation in each group must belong to two groups in order to communicatebetween groups. The method defines a coordinated approach in which allstations are able to communicate with each other without changing groupand/or a new frequency pattern associated therewith.

A clear frequency pattern is to be recognized with the use of themethod. At recurring intervals, a particular frequency is used for aparticular duration. Corresponding frequency patterns can be defined inthe system standard in order to determine common coordination times.Alternatively, corresponding algorithms or generator polynomes forgenerating patterns can be defined in the system standard. In order thatan exchange between stations with different frequency patterns is madepossible, all participating stations will use this common frequencychannel after a particular time. In order to limit delays, this durationwill for example lie within the range of a MAC frame (MAC—Medium AccessControl). During these phases, in which a particular frequency is beingused, information for organizing the radio medium is exchanged orsignaled as to which frequencies are busy (status information) and whichfrequencies are to be occupied in future (reservation requests).

One particular feature is the introduction of an FDMA method for theefficient use of a large number of frequency channels at any time,without making any particular demands on the stations' radio hardware.The method will preferably be employed in decentrally organized systems.This involves a station periodically switching between two phases whichdefine a frequency pattern. During one phase, it transmits and receiveson a predefined frequency used for organizing the radio medium. Duringthe other phase, a station is free to select any frequency channel. Byintroducing at least three frequency patterns, it is possible to selectany frequency channel at any time depending on which of the possiblefrequency patterns is chosen.

Initially, to avoid reservation conflicts and to ensure the availabilityof adequate resources, all the transmission resources for communicatingbetween stations within a group are used during those times which arecapable of being used exclusively by this group alone. These times occurif according to the example in FIG. 2, two other groups happen to beexchanging data over the coordination frequency. As far as possible thephase during which all stations are free to use the frequencies is usedonly for communicating between two groups. The shared coordination phasecan also be used for exchanging data between two groups. For multicastand broadcast, that phase is used during which all stations are free totransmit on the frequencies. The method can also be applied in the samemanner to centrally organized systems. The advantage then resides in thefact that the central entity need be no more complex than the otherstations. This means that the central role, as for example withHIPERLAN/2 or Bluetooth, could be transferred dynamically to anystation.

The invention has been described in detail with particular reference topreferred embodiments thereof and examples, but it will be understoodthat variations and modifications can be effected within the spirit andscope of the invention covered by the claims which may include thephrase “at least one of A, B and C” as an alternative expression thatmeans one or more of A, B and C may be used, contrary to the holding inSuperguide v. DIRECTV, 69 USPQ2d 1865 (Fed. Cir. 2004).

1-5. (canceled)
 6. A method for transmitting signals in a radiocommunication system having at least first and second frequency channelsfor signal transmission, comprising: using at least the first frequencychannel for organizing radio resources; using at least the secondfrequency channel for transmitting signals between stations of thesystem; and using a predefined temporal pattern for signal transmissionfrom/to stations on the frequency channels.
 7. The method according toclaim 6, wherein stations are assigned station specific patterns forusing the frequency channels.
 8. The method according to claim 6,wherein the first frequency channel is used to transmit at least one ofcontrol messages, information relating to protocol operations andinformation related to administration of radio resources.
 9. The methodaccording to claim 6, wherein the radio communication system is adecentrally organized system.
 10. The method according to claim 7,wherein the first frequency channel is used to transmit controlmessages, information relating to protocol operations and informationrelated to administration of radio resources.
 11. The method accordingto claim 10, wherein the radio communication system is a decentrallyorganized system.
 12. A station of a radio communication system, withthe means to perform the method according to claim
 11. 13. A station ofa radio communication system, with the means to perform the methodaccording to claim 6.